If you haven't already, try exporting them as a .png file and changing the .material coding accordingly.
It doesn't matter if you export it for them because then your accounts might be linked when you go to put in their username to export it correctly.
Anyways. Make sure the files are .png as said above and check the material file. If you want we could look over it ourselves and it might be easier to point out the problem.
If you want to do that, copy and past the material file here so we can see it and show us what you saved the files as; png or jpg.